I agree 100% with Ogden. A hybrid is a good place to start to see if you like cycling. You can usually get them for around $500-700. If you have a performance bike shop they always have sales. Try checking Craigslist as well. Biggest thing is the tires. Get a rim with 32spokes if you are concerned with damaging them. I was 300lbs when I started riding last year, and never had a problem. I even started, and fell in love, with a road bike. Just make sure tire pressure stays high and you should be fine. A minimum of 120psi should work. If you feel you need more ask the local shop for tires that are rated to 140 or even 160psi. Good luck and have fun.
sean